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Acid pretreatment of wood provides significant energy savings during refining but reduces the brightness of the pulp. Acid treatments also extract carbohydrates from wood. Addition of an acid pretreatment process to a thermomechanical pulping process therefore offers an opportunity to reduce refining energy cost and provide a secondary product from a fermentation ethanol plant. A process being investigated by BioPulping International and the Forest Products Laboratory involves pretreatment with oxalic acid or diethyl oxalate and offers 25% or more reduction in specific refiner energy consumption, with a minor sacrifice in brightness. This treatment also results in extraction of approximately 6% of the wood mass. Similarly, research during the late 1970s on sulfonated chemimechanical pulping at low pH determined that bisulfite reduced specific refining energy, maintained brightness, and released carbohydrates. The similarity in behavior of these two pretreatments suggests a common mechanism that is the subject of this study. Our hypothesis is that both acids provide optimal conditions, either buffering at pH 2 or mildly reducing conditions, for pretreating wood. To test this hypothesis, a series of spruce and aspen veneer samples were pretreated with sodium bisulfate, sulfurous acid, and oxalic acid. These three acids can provide buffering near pH 2 and a range of redox potential. The wood chip brightness of the sodium bisulfate and oxalic acid experiments were similar but at a given yield sulfurous acid seems to preserve brightness better than does either bisulfate or oxalic acid. The redox activity does not seem to affect results.